The invention originates with a commutation device for a direct-current motor, comprising among other things collector bars and brushes according to the preamble to the main claim, for which the current ripple can be detected with the aid of an electronic circuit and can be evaluated as measure for the speed or the number of motor rotations. The detection, evaluation and use of the alternating component of the current ("ripple") for direct-current motors as measure for the motor speed is generally known. One method used in this connection is the so-called ripple-count method. With this method, the analog signal of the commutation current proceeds in addition to the higher-frequency ripple with a low-frequent modulation due to production tolerances, caused by differences in the coil sections of the motor armature. In the process, local minima and maxima occur, for which the minima can have absolute higher values than the maxima. For that reason, a local detection by means of an involved, differentiating method is necessary.
The problem of evaluating the analog signal of the commutation current at low cost and with low expenditure for the circuit technology is partially solved with the aid of a known method and arrangement in the DE-A-44 22 083 (German Published Examined Application).
However, a slight geometric displacement of the jointly operating brushes in particular will lead to additional current ripples in the signal course of the commutation current and thus to undesirable false signals since the shape of the brush running surface considerably influences the analog signal of the commutation current.
A known device for improving the analog signal of the commutation current (reference DE-A-31 48 966) represents one such attempt to solve this problem and avoid false signals. In this reference, it is explained that the improved analog signal is the result of a change in the shape of the brush running surface, which has two essentially point-shaped or line-shaped contact faces to the commutator, thus leading to a better coverage with respect to time and geometry, as well as a longer commutation time.
One disadvantage of these arrangements is the loud brush noise caused by the collector bars being run abruptly under the brush. The brush noise is defined, in a manner known per se, as the airborne sound generated in the process, which can be measured with a microphone. With the modified shape of the brush running surface according to the DE-A-44 38 868, the collector bars in each case are run twice per brush under the brush running surface, thereby causing and even stronger noise development.